The biuretization of isocyanates is known in the art. U.S. Pat. Nos. 3,903,127 and 3,976,622 describe a variety of different biuretizing agents, including primary aliphatic amines. Canadian published application 2,211,025 describes the use of tertiary alcohols or a mixture of water and a tertiary alcohol to biuretize an isocyanate. Finally, U.S. Pat. No. 4,220,749 describes the use of secondary monoamines as biuretizing agents. All three references describe the use of hexamethylene diisocyanate as the starting isocyanate. U.S. Pat. No. 4,788,262 suggests biuretizing a trimer of hexamethylene diisocyanate, but only exemplifies a mixture of trimers and biurets (see also U.S. Pat. No. 6,133,397),
The use of polyisocyanates curing agents for coating compositions is also known, with polyisocyanates having isocyanate functionalities of 3 or higher being particularly preferred. U.S. Patent Application Publication 2003/0109664 describes the production of a higher functional polyisocyanate by biuretizing an isocyanaurate-group containing polyisocyanate. Among the starting isocyanates described is a trimer of hexamethylene diisocyanate. The biuretizing agent used is water. The application indicates that when compared to isocyanates biuretized with t-butanol or a mixture of t-butanol and water, the trimer-containing isocyanates biuretized with water only, exhibited improved color. The application also broadly suggests that the isocyanate groups of biuretized isocyanates can be blocked with alcohols, ketimines or oximes. Although the biuretized isocyanates described in the application are an improvement over the prior art biuretized isocyanates, it would be desirable to improve the gel content of coatings prepared from such isocyanates.
A wide variety of blocking agents are known in the art (see, e.g., “Blocked Isocyanates in Coatings,” Potter et al, presented at the Water-Borne & Higher-Solids Coatings Symposium, New Orleans, February 1986. Among the blocking agents described are i) phenol, cresols and long aliphatic chain substituted phenols (such as isononylphenol), ii) amides (such as ε-caprolactam), iii) oximes (such as butanoneoxime), iv) active methylene group-containing compounds (such as malonates and aectoacetates) and v) sodium bisulfite. Various blocking agents are also described, e.g., in U.S. Pat. Nos. 4,324,879, 4,439,593, 4,495,229, 4,518,522, 4,667,180, 5,071,937, 5,705,593, 5,780,541, 5,849,855, 6,051,675, 6,060,573, 6,274,693, 6,368,669 and 6,583,216.
More recently, secondary amines such as N-benzyl-tert-butylamine (published European patent application 1,375,549, corresponding to U.S. patent application Ser. No. 10/459,033, filed on Jun. 10, 2003) and 3-tert-butylamino methyl proprionate (U.S. patent application Ser. No. 10/874,716, filed Jun. 23, 2004) have been described as useful blocking agents.